On helicopters, two aspects have to be particularly cared for. On the one hand, a counter torque has to be provided to the torque of the horizontal rotor driven for rotation about the vertical rotor axis. Without such a counter torque the fuselage of the helicopter would rotate about the vertical rotor axis in opposite direction to the horizontal rotor.
On the other hand, in forward flight of a helicopter, due to the rotational movement of the horizontal rotor, the flow against the rotor blades on the advancing blade side of the horizontal rotor strongly differs from the flow against the rotor blades on the retreating blade side of the horizontal rotor. On the advancing blade side, the rotor blades move faster in forward direction than the fuselage of the helicopter and are thus subject to a correspondingly higher relative flow velocity of the air surrounding of the helicopter. As a result, the rotor blades on the advancing blade side of the horizontal rotor provide a comparatively high lift even at small pitch angles.
On the retreating blade side of the horizontal rotor, however, the rotor blades, relative to the fuselage of the helicopter, move opposite to the direction in which the entire helicopter flies forward. With same pitch angles, this results into a considerably lower speed of the flow against the rotor blades with correspondingly lower lift on the retreating blade side. In the inner area of the horizontal rotor, where the circumferential velocities of the rotor blades are lower, even reverse flow may occur at the rotor blades, which result in a negative or downward lift.
It is a known method to provide a further horizontal rotor driven in opposite direction for providing the counter torque to the torque of the horizontal rotor. A more common measure for providing the counter torque to the torque of the horizontal rotor, however, is a vertical tail rotor driven for rotation about a horizontal axis at a tail of the helicopter. In known helicopters, this tail rotor is driven at a fixed gear ratio together with the horizontal rotor. Different torques of the horizontal rotor of the helicopter are then compensated for by different pitch angles of the rotor blades of the tail rotor. A tail rotor may also be shrouded and is then designated as a fenestron. The shroud of the tail rotor has the result that its rotor blades are not subject to an undesired radial flow at higher forward velocities of the helicopter. Further, the shroud of the tail rotor reduces the danger of colliding with its rotating rotor blades which are hardly visible to people approaching the landed helicopter.
In forward flight of a helicopter, the tail rotor may be assisted by a fin at the end of the tail, and the tail rotor may even become needless due to this fin. Such a fin at the end of the tail also keeps a helicopter stabilized in its flight direction when its drive fails and when its horizontal rotor is excited for autorotation by the flow due to the sinking speed of the helicopter.
The different flow velocities of the flow against the rotor blades on the advancing blade and the retreating blade side of the horizontal rotor of a helicopter are generally coped with in that the pitch angle of the rotor blades is varied over each rotation about the vertical rotor axis of the horizontal rotor periodically between a low pitch angle on the advancing blade side and a high pitch angle on the retreating blade side of the horizontal rotor. Nevertheless, the different flow velocities on the advancing blade and the retreating blade side of the horizontal rotor mark a limit for the velocity reachable with a helicopter in forward flight as the maximum pitch angle on the retreating blade side is limited by the danger of a flow separation with a simultaneous breakdown of the lift.
The Eurocopter X3 is known as a helicopter for higher forward velocities. This helicopter is a hybrid gyrocopter which has two short wings laterally attached to its fuselage below its horizontal rotor, each wing being provided with a large propeller directed in forward direction. In addition to increasing the forward drive of the helicopter, these propellers also provide the counter torque to the torque of the horizontal rotor. The Eurocopter X3 comprises a passive tail with a double fin. With higher forward velocities, the rotational speed of the horizontal rotor is lowered to avoid transonic flow velocities of the rotor blades. Then, the required lift is essentially provided by the short wings.
The known helicopter Lockheed AH-56 comprises short wings laterally attached to its fuselage below the horizontal rotor and, in addition to the usual tail rotor, an additional thrust propeller at its tail for reaching higher flight velocities.
The known helicopter Sikorsky Piasecki X-49 comprises short wings laterally attached to its fuselage below the horizontal rotor, and a shrouded thrust propeller at its tail. The airflow created by this thrust propeller is laterally deflectable with flaps to provide the counter torque to the torque of the horizontal rotor.
With all helicopters comprising short wings below the horizontal rotor, there are strong mechanical loads to the short wings during hovering flight of the respective helicopter. In hovering, the horizontal rotor propels air from the surroundings downwardly to generate lift. The resulting downward airflow hits the short wings from above. Further, this airflow is not homogeneous, but it comprises vortices and even shockwaves which cause considerable accelerations of the short wings in different directions.
JP 10181694 A discloses helicopter comprising a horizontal rotor, a tail with a stab, a fin and a tail rotor, and a further boom on the retreating blade side of the horizontal rotor for selectively generating lift here in forward flight of the helicopter. The lateral boom can be swiveled about a vertical swivel axis from beneath the fuselage of the helicopter in forward direction until it extends perpendicular to the vertical longitudinal middle plane of the helicopter. Additionally, the pitch angle of the boom may be varied in that the boom is swiveled about its main axis extending away from the rotor axis of the horizontal rotor. This second swivellability is used to vary the lift generated by the boom.
JP 7132893 A discloses a helicopter which, besides a horizontal rotor and a tail with a tail rotor, comprises booms on both the advancing blade and the retreating blade sides of the horizontal rotor in a symmetric arrangement with regard to the vertical longitudinal middle plane of the helicopter. The booms are selectively swiveled about a swivel axis running parallelly to the longitudinal middle plane of the helicopter, to generate a higher lift on the retreating blade side than on the advancing blade side of the horizontal rotor during forward flight of the helicopter. Further, the booms can be swiveled in such a way that their leading edges point upwards and their trailing edges point downwards to minimize their cross-section facing the horizontal rotor during hovering flight of the helicopter. Such a helicopter configuration is also known from DE 1 756 222 C, corresponding to U.S. Pat. No. 3,464,651, JP 5229490 A and DE 2 236 415 A, corresponding to GB 1 394 177.
FR 1 467 573 A discloses a helicopter comprising a horizontal rotor and a tail with a tail rotor. Additionally, a single stab is provided at the tail on the retreating blade side of the horizontal rotor. Further, this known helicopter comprises a telescopically extendable boom on the retreating blade side of the horizontal rotor for selectively generating lift on this side of its vertical longitudinal middle plane.
U.S. Pat. No. 2,580,312 A discloses a helicopter which, besides a horizontal rotor and a tail with a fin and a stab, comprises wings which may be folded in and a backward oriented propeller.
DE 1 581 060 B discloses a helicopter with a tail rotor which can be swiveled about a vertical swivel axis.
DE 1 144 116 A discloses a helicopter with a tail comprising a tail rotor. A variable pitch angle of the rotor blades of the tail rotor is controlled together with a fin also provided at the tail.
DE 41 19 388 A1 discloses a helicopter with a side rotor. The side rotor is a vertical rotor which is provided on the advancing blade side of the horizontal rotor of the helicopter, and which is mounted to the tail of the helicopter via a boom extending perpendicular to the vertical longitudinal middle plane of the helicopter.
DE 1 118 017 A discloses a helicopter with a tail rotor which is shrouded and whose airflow can be deflected by means of flaps which may be swiveled about vertical axes.
US 2011/0036954 A1 discloses a helicopter comprising a horizontal rotor, wings arranged below the horizontal rotor and a backward oriented propeller at a tail. This propeller is shrouded, and the airflow generated by the propeller which is generally directed backwards can be laterally deflected by means of a flap.
US 2011/0024551 A1 discloses a helicopter comprising a horizontal rotor, wings extending perpendicular to the vertical longitudinal middle plane of the helicopter, at which vertical rotors are arranged, and a short tail provided with a fin and a stab.
There still is the need for a helicopter which, despite a particularly simple configuration, allows for reaching high flight velocities.